Multotec specialises in iron ore beneficiation equipment with over 45 years of global experience and leading technology. Our team of metallurgists and engineers will partner with you to customise your equipment, helping your process run more efficiently while lowering your overall cost per ton. All Multotec equipment used in this process is shown in green on this flowsheet and is for illustrative purposes only; click on the red pins to find out more about each product, download PDF orcontact usfor more information.
Multotec specialises in iron ore beneficiation equipment with over 45 years of global experience and leading technology. Our team of metallurgists and engineers will partner with you to customise your equipment, helping your process run more efficiently while lowering your overall cost per ton.
All Multotec equipment used in this process is shown in green on this flowsheet and is for illustrative purposes only; click on the red pins to find out more about each product, download PDF orcontact usfor more information.
Beneficiation of Iron Ore and the treatment of magnetic iron taconites, stage grinding and wet magnetic separation is standard practice. This also applies to iron ores of the non-magnetic type which after a reducing roast are amenable to magnetic separation. All such plants are large tonnage operations treating up to 50,000 tons per day and ultimately requiring grinding as fine as minus 500-mesh for liberation of the iron minerals from the siliceous gangue.
Magnetic separation methods are very efficient in making high recovery of the iron minerals, but production of iron concentrates with less than 8 to 10% silica in the magnetic cleaning stages becomes inefficient. It is here that flotation has proven most efficient. Wet magnetic finishers producing 63 to 64% Fe concentrates at 50-55% solids can go directly to the flotation section for silica removal down to 4 to 6% or even less. Low water requirements and positive silica removal with low iron losses makes flotation particularly attractive. Multistage cleaning steps generally are not necessary. Often roughing off the silica froth without further cleaning is adequate.
The iron ore beneficiation flowsheet presented is typical of the large tonnage magnetic taconite operations. Multi-parallel circuits are necessary, but for purposes of illustration and description a single circuit is shown and described.
The primary rod mill discharge at about minus 10- mesh is treated over wet magnetic cobbers where, on average magnetic taconite ore, about 1/3of the total tonnage is rejected as a non-magnetic tailing requiring no further treatment. The magnetic product removed by the cobbers may go direct to the ball mill or alternately may be pumped through a cyclone classifier. Cyclone underflows usually all plus 100 or 150 mesh, goes to the ball mill for further grinding. The mill discharge passes through a wet magnetic separator for further upgrading and also rejection of additional non-magnetic tailing. The ball mill and magnetic cleaner and cyclone all in closed circuit produce an iron enriched magnetic product 85 to 90% minus 325 mesh which is usually the case on finely disseminated taconites.
The finely ground enriched product from the initial stages of grinding and magnetic separation passes to a hydroclassifier to eliminate the large volume of water in the overflow. Some finely divided silica slime is also eliminated in this circuit. The hydroclassifier underflow is generally subjected to at least 3 stages of magnetic separation for further upgrading and production of additional final non-magnetic tailing. Magnetic concentrate at this point will usually contain 63 to 64% iron with 8 to 10% silica. Further silica removal at this point by magnetic separation becomes rather inefficient due to low magnetic separator capacity and their inability to reject middling particles.
The iron concentrate as it comes off the magnetic finishers is well flocculated due to magnetic action and usually contains 50-55% solids. This is ideal dilution for conditioning ahead of flotation. For best results it is necessary to pass the pulp through a demagnetizing coil to disperse the magnetic floes and thus render the pulp more amenable to flotation.
Feed to flotation for silica removal is diluted with fresh clean water to 35 to 40% solids. Being able to effectively float the silica and iron silicates at this relatively high solid content makes flotation particularly attractive.
For this separation Sub-A Flotation Machines of the open or free-flow type for rougher flotation are particularly desirable. Intense aeration of the deflocculated and dispersed pulp is necessary for removal of the finely divided silica and iron silicates in the froth product. A 6-cell No. 24 Free-FlowFlotation Machine will effectively treat 35 to 40 LTPH of iron concentrates down to the desired limit, usually 4 to 6% SiO2. Loss of iron in the froth is low. The rough froth may be cleaned and reflotated or reground and reprocessed if necessary.
A cationic reagent is usually all that is necessary to effectively activate and float the silica from the iron. Since no prior reagents have come in contact with thethoroughly washed and relatively slime free magnetic iron concentrates, the cationic reagent is fast acting and in somecases no prior conditioning ahead of the flotation cells is necessary.
A frother such as Methyl Isobutyl Carbinol or Heptinol is usually necessary to give a good froth condition in the flotation circuit. In some cases a dispersant such as Corn Products gum (sometimes causticized) is also helpful in depressing the iron. Typical requirements may be as follows:
One operation is presently using Aerosurf MG-98 Amine at the rate of .06 lbs/ton and 0.05 lbs/ton of MIBC (methyl isobutyl carbinol). Total reagent cost in this case is approximately 5 cents per ton of flotation product.
The high grade iron product, low in silica, discharging from the flotation circuit is remagnetized, thickened and filtered in the conventional manner with a disc filter down to 8 to 10% moisture prior to treatment in the pelletizing plant. Both the thickener and filter must be heavy duty units. Generally, in the large tonnage concentrators the thickener underflow at 70 to 72% solids is stored in large Turbine Type Agitators. Tanks up to 50 ft. in diameter x 40 ft. deep with 12 ft. diameter propellers are used to keep the pulp uniform. Such large units require on the order of 100 to 125 HP for thorough mixing the high solids ahead of filtration.
In addition to effective removal of silica with low water requirements flotation is a low cost separation, power-wise and also reagent wise. Maintenance is low since the finely divided magnetic taconite concentrate has proven to be rather non-abrasive. Even after a years operation very little wear is noticed on propellers and impellers.
A further advantage offered by flotation is the possibility of initially grinding coarser and producing a middling in the flotation section for retreatment. In place of initially grinding 85 to 90% minus 325, the grind if coarsened to 80-85% minus 325-mesh will result in greater initial tonnage treated per mill section. Considerable advantage is to be gained by this approach.
Free-Flow Sub-A Flotation is a solution to the effective removal of silica from magnetic taconite concentrates. Present plants are using this method to advantage and future installations will resort more and more to production of low silica iron concentrate for conversion into pellets.
During the last ten years great strides have been made, on the Mesabi range, in the practice of beneficiating low grade iron ore material. By beneficiation is meant all methods of removing impurities, and raising the iron content to a point where it can be sold in open market, the principal impurities being silica and moisture. The general processes to which low grade iron ores are amenable are as follows:
(a) Drying; removes hydroscopic or atmospheric moisture. (b) Calcining; removes carbon dioxide from iron carbonate, molecular water from hydrated hematites, and atmospheric moisture. (c) Roasting; removes sulphur, carbon dioxide, molecular water and atmospheric moisture. (d) Agglomeration; primarily for the purpose of preparing finely divided material for blast furnace; briquetting and sintering.
(a) Screen sizing; removes rock and sand. (b) Classification; removes sand by means of currents of water of varying velocities. (c) Log washing; removes fine sand. (d) Jigging; removes larger particles of impurities than is possible by log washing. Certain types of jigs remove fine sand. (e) Reciprocating tables; recover fine iron particles from sand discarded by above processes. (f) Magnetic separation; applicable to the commercial separation of the magnetic oxide of iron from gangue material. From a scientific standpoint it is possible to separate certain hematites and limonites from their gangue. (g) Miscellaneous processes; comprise dry concentration, electro-static separation and other processes.
At this time four of the above methods of beneficiation, i. e., Drying, Screening, Log Washing, and Table Concentration are in use, and a plant for a fifth process, magnetic concentration, is now under construction.
The Brunt Ore Drying Plant. In 1910, M. A. Hanna & Co. built an experimental drying plant at the Hollister mine, Crystal Falls, Mich. This was the first plant of this nature built in the iron districts of Lake Superior. The ore to be dried was of very painty nature, being high in alumina, and the moisture content was in the neighborhood of 18 to 20 per cent. The capacity of this plant was approximately twenty tons per hour. The type of dryer used was what is known as the Cummers Patent Dryer. Considerable tonnage was handled through this plant with satisfactory results. By eliminating a part of the moisture the percentages of the other ingredients of the ore are raised in proportion to the amount of moisture abstracted. It was found by drying the iron ore that such grades as were a few points below the required iron content could be raised to grades that would be accepted as merchantable orein other words, taken out of the lean ore class. Many difficulties were encountered at the Hollister plant in the mechanical handling of the materials. As iron ore is of an extremely abrasive nature, the wear and tear on conveyors, elevators, chutes and any other apparatus with which it came in contact was very excessive. This plant was finally dismantled, but not until the ore at the property had been shipped.
In 1911, the above company began building a dryer of much larger proportions at the Brunt mine, Mountain Iron, Minn. The capacity of this plant was to be 120 tons per hour. The first year of its operation only two units were installed, each having a capacity of forty tons per hour. These also were the Cummers Dryers, and were designed especially for iron ore. A great many mechanical difficulties were enconutered in this installation, due to the stickiness and the high moisture content of the ore to be handled. After the first years, operation it was found necessary to practically tear down the whole plant and rebuild it along different lines, as the experience gained in the first years operation proved that the equipment was entirely inadequate to cope with the situation. The wet ore bins were changed; the feeders required an entirely different arrangement. To take the place of conveyor belt, a bucket conveyor was installed, and the whole plant was changed over from steam driven equipment to individual electric motor drives. This same year two more dryer units were installed. These were of the Ruggles-Cole type. Each of these machines had a capacity of twenty tons per hour. Again an endless number of mechanical troubles were encountered with this equipment, the gear drives, etc., being entirely too light for the work imposed upon them. These were enlarged and rebuilt. The wear and tear on this equipment was very severe, and these dryers were practically rebuilt throughout with the exception of the outside shells. At this time large dust collectors were installed for collecting the dust which was carried over by the fans from the dryers. The theory was that the ore should be dried down to a moisture content of 4 or 5 per cent. The first cargoes shipped to the lower lake ports gave a great deal of trouble due to the dust which was given off during the handling, and finally the moisture content of dried ore was increased to between 8 and 11 per cent. When the ore was dried to this moisture content very little dust was given off from the dryers; consequently the use of dust collectors was discontinued, the fans discharging directly to the atmosphere through short stacks. The continuous bucket conveyor was finally discarded on account of the high cost of upkeep and delays caused by breakage and repairs. This was taken out and replaced by continuous bucket elevators which are giving very satisfactory results to date.
The ore that is being dried at the Brunt mine contains from 16 to 22 per cent moisture and is reduced to about 9 or 10 per cent moisture content. The furnace men are very well pleased with the structure of the ore after it has been dried, as in drying it nodulizes or rolls into small pellets, which makes a very satisfactory furnace ore. At the same time that the dryer was being built at the Brunt mine, the Shenango Furnace Company installed a large plant at the Whiteside mine, at, Buhl. This plant consisted of four large Ruggles-Cole type dryers, each having a capacity of thirty tons per hour. The location for the Whiteside dryers was ideal as the ore was transferred through the plant entirely by gravity, no conveyors, elevators or other ore handling equipment being necessary outside of bins and feeders. The Whiteside plant experienced considerable difficulty similar to the Ruggles-Cole units at the Brunt, the gear drives, etc., being too light in design to stand up under the severe work. A new driving mechanism was finally installed at this
In iron ores which have a natural iron content of 45 to 48 per cent, and a moisture content of 14 to 20 per cent, an abstraction of 2 per cent moisture will increase the iron content approximately 1 per cent. Thus, if from an iron ore containing 48 per cent iron, as mined, 6 per cent moisture be abstracted, it yields a product of approximately 51 per cent iron. Considerable money has been spent in developing the drying of iron ore, and the results have been very satisfactory.
Trout Lake Concentrator. The first attempt to wash Mesabi ores was made in 1901-2, when a carload of ore was sent from the Arcturus property to Cedartown, Ga. The results justified further investigation, and in 1903-4 a small experimental plant was built at the Holman mine. In 1905, the Oliver Iron Mining Company became interested, and after exhaustive investigations conducted during the following four years erected the present Trout Lake concentrator. The work was commenced in April, 1909, and the plant was ready for use in 1910. It is located on the east side of Trout Lake, readily accessible from all directions. The mill building is of heavy steel construction throughout, 255 feet long, 162 feet wide, and 124 feet high, enclosed with corrugated iron. The approach to the mill is an earth fill, some 4,000 feet long, containing several million cubic yards of stripping from the Canisteo and Walker pits. It has a maximum height of 125 feet and was planned to accommodate four tracks. A steel trestle, 650 feet long, connects it with the mill. At the opposite end of the mill, 300 feet of additional steel work is in place and is now being used for tail track; it can be utilized for an addition to the mill if need arises.
is pumped direct to a 100,000-gallon supply tank at the mill. All the machinery in the mill is electrically driven. A 1,250 k.v.a. direct-connected generator transmits electric power at 6,600 volts to the mill, where it is stepped down to 440 volts.
The total amount of steel in the mill building, approach, tail-track and power plant structures is approximately 7,000 tons. The total cost of mill and equipment, according to the published report of the United States Steel Corporation, approximates $1,500,000.
The concentrating machinery is arranged in live units, each complete and capable of independent operation. This was done in order to keep the machines within reasonable size, to be able to handle separately the ores from different properties at the same time, and to increase the capacity. The tail-track already mentioned is long enough for seven additional units. The crude ore from the different mines is hauled to the mill over the big fill approach and trestle and dumped into the bins at the top of the mill. Each unit has a separate bin of 450 to 500 tons capacity. The ore is sluiced out from the bottoms of the bins by a hydraulic jet, and descends by gravity through the different machines. The crude ore tracks are 90 feet above the concentrate tracks. The ore is handled entirely by gravity; there is no elevating machinery to get out of order, other than the sand pumps necessary to lift the table concentrates to dewatering tanks over the concentrate bins.
One crude ore bin and one bar-grizzly. One 20-foot conical revolving trommel with 2-inch openings. One picking-belt, to receive and convey the trommel oversize to the concentrate bin. (The taconite chunks are picked off the belt by hand and dropped into a rock bin.) Two 25-foot log washers, one on either side of the trommel to treat the trommel throughs, discharging a product into the concentrate bin. Two chip-screens, one for either log, receiving log overflow
Two settling tanks, receiving chip-screen throughs. Two 18-foot turbos (small log washers to treat fines), receiving the tank settlings, discharging a concentrate directly into the concentrate bin. Four settling tanks receiving the overflow from turbo and first settling tanks. Twenty Overstrom tables, arranged in two rows of ten each, to treat the settlings from the four settling tanks just mentioned. Eight Frenier spiral sand pumps (four primary and four relay), to pump the table concentrates to dewatering tanks from which they discharge directly into the concentrate bin.
The mill product thus consists of belt product, log-, turbo-, and table-concentrates. Each unit has its own concentrate bin of ninety tons capacity. Tracks for ore cars run directly under the bins, a separate track serving each bin. The cars are stored in a yard south of the mill, from which they can be spotted under the bins by releasing the brakes, as the grade from the yard is down towards the mill. The tailings consist of chip-screen discharge, No. 2 and No. 3 settling-tank overflow and table tailings. They are collected by launders in the mill basement and discharged into a concrete launder outside the table addition to mill. This launder discharges into Trout Lake, some 2,000 feet distant. The amount of water used per unit is about 1,500 gallons per minute, or 90,000 gallons per hour, approximately 360 gallons per ton of concentrates.
The rock picked off the belt and bar-grizzly is drawn from the rock pockets or bins into a rock car and hauled by an electric motor to a stockpile east of the mill. One motor and car serves all five units. One 100-h.p. electric motor drives the trommel, picking belt, logs and turbos; one 15-h.p. motor drives the tables and chip-screens; one 20 h.p. motor runs the sand pumps in the basement of the mill.
In 1910, the mill was operated on two eleven-hour shifts. In 1911, this was cut down to two ten-hour shifts. The average capacity is nearly 400 tons of crude ore per hour per unit, the maximum being 900 tons. The largest tonnage washed in one season of six months was over 4,000,000 tons of crude ore. The mill can not be operated in freezing weather, and its operating season coincides with the ore-shipping season. Nearly 100 men are employed on the day shift, and 75 to 80 on the night shift.
The average grade of crude ore varies greatly, depending on the character of material being treated and the local conditions at the time of mining. The concentrate product varies within wide limits, depending upon the character and class of ore, just as the grade of direct-shipping ores varies greatly. This is not a matter of degree of concentration produced by the mill, but rather the amount of concentration and enrichment produced by nature.
The results obtained vary widely for different ore bodies and even for different layers in the same ore body. The quantity of rock sorted out in the pit is so variable that the mere statement of mill recovery might give a very misleading idea of the total percentage of recovery from these low grade ore bodies. It may be authoritatively said that the mill has satisfactorily solved the problem of economical handling of western Mesabi ore bodies, a most important achievement in the line of conservation of our national resources.
The Concentration of the East Mesabi Magnetites. The last method of treatment to be considered, and the last to make its appearance in the Lake Superior district, is magnetic concentration. This method of beneficiating iron ores has been in use for many years in New-York State and Canada, and some foreign countries, but, until recently, it has not been used in this district.
The whole process of magnetic concentration as applied to the eastern Mesabi magnetites is a good illustration of the manner in which the various types of machines can be made to work together so as to produce a high-grade furnace product from an ore material containing only 25 per cent of iron in the form of magnetite. The hard rock is first crushed to about three-inch size and is then passed over a magnetic cobber. The field strength of this cobber is so adjusted that all of the coarse material containing no magnetic iron is discarded as tailing. The concentrate from this cobber is still too low grade to be useful, and is, therefore, crushed again to two-inch size. This material is passed over a second cobber and the worthless gangue again discarded. This process of crushing, cobbing and discarding worthless material continues until the product has been reduced to about -inch size. When this stage has been reached, approximately one-half the ore has been discarded as tailing and the other half contains practically all of the magnetic oxide that was originally present in the rock. This -inch material, however, still contains too much gangue to be considered a desirable furnace product. It is, therefore, ground wet in ball mills until it will all pass a 100-mesh screen. This fine material is concentrated by magnetic log-washers in which the final separation is made. The concentrate produced by these machines is then dewatered by the use of continuous filters in the tank of which the fuel for sintering is mixed. The filter cake is conveyed directly to the sintering plant, where the ore is agglomerated. After being sintered the ore is screened in order to remove any fine material, and only the clean coarse sinter is shipped to the furnaces.
Rio Tinto Iron Ore's low-grade ore beneficiation plant in the Pilbara was commissioned in 1979. Initial engineering, design, and construction were undertaken by KBR (Kellogg Brown and Root) and Minenco (RTIO information provided to author, 2013). The plant separates closed-circuit crushed ROM into 31.5+6.3mm and 6.3+0.5mm streams for feeding their DMS drum and cyclone plants, respectively (Figure 10.5).
To evaluate an iron ore resource, develop processing routines for iron ore beneficiation, and understand the behavior of the ore during such processing, extensive mineralogical characterizations are required. For calculating mineral associations, mineral liberation, grain size and porosity distribution, and other textural data, reliable imaging techniques are required.
Automated optical image analysis (OIA) is a relatively cheap, robust, and objective method for mineral and textural characterization of iron ores and sinters. OIA allows reliable and consistent identification of different iron oxide and oxyhydroxide minerals, e.g., hematite, kenomagnetite, hydrohematite, and vitreous and ochreous goethite, and many gangue minerals in iron ore and different ferrites and silicates in iron ore sinter. OIA also enables a distinction to be made between forms of the same mineral with differing degrees of oxidation or hydration.
To reliably identify particles and minerals during OIA, a set of comprehensive procedures should be automatically applied to each processed image. Generally, this includes next stages: image improvement, particle and mineral identification, particle separation, porosity identification, identification of unidentified areas, and correction of mineral maps. This is followed by automated measurements of final mineral maps and statistical processing of results.
High resolution, imaging speed, and comprehensive image analysis techniques of modern OIA systems have made it possible to significantly reduce the cost and subjectivity of iron ore and sinter characterization with a simultaneous increase in the accuracy of mineral and textural identification.
World demand for iron ores to meet the ever-increasing requirements of iron and steel industries has made it imperative to utilize all available resources including lean grade ores, mined wastes, processed tailings, and blue dust fines accumulated at mine sites. Most of such resources exist as finer particles, while lean-grade ores require fine grinding for liberation of associated gangue minerals. Hematite is the most abundant iron ore mineral present in available resources while the major impurities include silica, alumina, calcite, clay matter, and phosphorus. Conventional beneficiation processes such as flotation, electrostatic and magnetic separation, gravity methods and flocculationdispersion using chemical reagents to treat the finer iron ore resources often prove to be inefficient, energy-intensive, costly, and environmentally-toxic.
Why microbially mediated iron ore beneficiation? Any microbially induced beneficiation process will prove to be cost-effective, energy-efficient, and environment-friendly compared to chemical alternatives which use toxic chemicals. Microorganisms which find use in beneficiation are indigenously present in iron ore deposits, tailing dams, and processed wastes. Mining organisms inhabiting iron ore deposits are implicated in biomineralization processes such as hematite, magnetite, and goethite formation as well as their oxidationreduction, dissolution, and precipitation in mining environments. Similarly, gangue minerals such as silica, silicates, clays, calcite, alumina, and phosphates are often biogenically entrapped and encrusted in the hematitemagnetite matrix.
Autotrophic, heterotrophic, aerobic, and anaerobic microorganisms such as Acidithiobacillus spp., Bacillus spp., Pseudomonas, Paenibacillus spp., anaerobes such as SRB, yeasts such as Saccharomyces sp., and fungal species inhabit iron ore mineralization sites. Many such organisms find use in beneficiation processes because they are capable of bringing about surface chemical changes on minerals. Microbial cells and metabolic products such as polysaccharides, proteins, organic and inorganic acids can be used as reagents in mineral flotation and flocculation.
Isolation, characterization, and testing the usefulness of mining microorganisms inhabiting iron ore deposits hold the key towards development of suitable biotechnological processes for iron ore beneficiation. Because many microorganisms inhabit iron ore deposits contributing to biogenesis and biomineralization, there is no reason why one cannot isolate and use them to bring about useful mineral processing functions. Though innumerable microorganisms are known to inhabit iron ore deposits, only a few of them have been identified as of now and among them, still only a few have been tested for possible iron ore beneficiation application.
Costly and toxic chemicals used in conventional beneficiation processes can be replaced by biodegradable, mineral-specific, biologically derived reagents such as exopolysaccharides, bioproteins, organic acids, biodepressants, and bioflocculants.
Iron ore beneficiation can be brought about through three approaches, namely, selective dissolution, microbially induced flotation, and selective flocculationdispersion. The bioprocesses are specially suited to treat fines, slimes, and waste tailings.
Potential applications includei.Dephosphorizationii.Desulfurizationiii.Desiliconizationiv.Alumina and clay removalv.Biodegradation of toxic mill effluentsvi.Clarification, water harvesting from tailing poundsvii.Recovery of iron and associated valuable minerals from accumulated ore fines and processed tailings.
For D. desulfuricans, an anaerobe, as the cell count increases, sulfate concentration decreases, because the organism reduces sulfate to sulfide to derive energy. During the log phase, the decrease in sulfate concentration corresponding to exponential bacterial growth was significant.
The growth of bacterial cells was monitored in the presence and absence of minerals such as hematite and quartz. When similar cell growth was attained in the presence of minerals as in control, growth adaptation to the minerals was considered achieved. Adsorption density of SRB cells grown under different conditions on hematite and quartz surfaces was found to be different. Cells grown in the presence of hematite exhibited higher adsorption density on hematite, whereas those grown in the presence of quartz attached profusely to quartz surfaces. Cells grown in the absence of minerals exhibited higher surface affinity towards hematite and rendered it more hydrophilic . Extracellular proteins and ECP secreted by D. desulfuricans in the presence and absence of minerals are shown in Table 10.19.
Extracellular proteins secreted by quartz-grown D. desulfuricans were the highest, while the secretion of ECP was found to be higher in case of hematite-grown cells. Bacterial growth in the presence of quartz promoted secretion of higher amounts of proteins, while the presence of hematite resulted in the generation of significant amounts of exopolysaccharides. Negatively charged quartz surfaces exhibit strong surface affinity towards positively charged amino group containing proteinaceous compounds, while hematite exhibited strong affinity towards exopolysaccharides at neutral to mildly alkaline pH conditions.
Protein profiles of bacterial cells and metabolites exposed to minerals were compared with conventionally grown cells and their metabolites. Mineral-specific protein bands of molecular weights 105, 36.5, and 25kDa were observed only in case of quartz-grown bacterial cells because they were absent in conventionally grown and hematite-adapted cells and metabolites. Secretion of higher amounts of mineral-specific stress proteins by bacterial cells was promoted if grown and adapted in the presence of quartz mineral .
Amount of polysaccharides present on hematite-adapted SRB cell walls as well as metabolites were significantly higher compared to bacterial growth in the presence of quartz. SRB cells adapted to hematite become more hydrophilic than those adapted to quartz, which were rendered more hydrophobic due to enhanced secretion and adsorption of proteins. Similarly, hematite surfaces were rendered hydrophilic due to enhanced polysaccharide adsorption, while quartz became hydrophobic due to higher protein adsorption.
Significant surface chemical changes brought about on quartz and hematite due to bacterial interaction can be made use of in their selective separation through bioflotation as illustrated in Table 10.20.
In the absence of bacterial interaction, no significant flotation of quartz and hematite would be possible. Percent weight flotation of quartz was about 45% and 35% after interaction with unadapted bacterial cells and metabolite, respectively, while it increased to about 75% and 84% on interaction with quartz-adapted cells and metabolite, respectively. Percent weight flotation of hematite was about 8% and 11% on interaction with unadapted bacterial cells and metabolite, respectively. After interaction with hematite and quartz-adapted bacterial metabolite, about 15% of hematite could be floated. Flotation recovery of hematite decreased to 2% with hematite-grown cells. Such a hydrophilic surface character of hematite (unlike quartz) is due to its high affinity towards polysaccharides.
Selective separation of quartz from a binary mixture of quartz and hematite was also studied after interaction with bacterial cells and metabolite. Interaction with unadapted bacterial cells and metabolite resulted in only 10% and 9% flotation recovery for hematite. After interaction with quartz-adapted bacterial cells and metabolite, the percent flotation of quartz from the mixture was about 76% and 81%, respectively. The above results clearly establish that efficient separation of silica from hematite could be achieved through selective flotation after interaction with cells and metabolites of an SRB (D. desulfuricans). However, prior bacterial adaptation to the respective minerals (especially quartz) is essential to bring about efficient separation. Addition of starving quantities of silica collector would be beneficial in enhancing quartz floatability and depression of hematite.
Uncertain parameters are assumed to behave like fuzzy numbers and FEVM approach has been applied to an industrial case study of ore beneficiation process. A modified form of NSGA II, FENSGA-II has been utilized to solve the deterministic equivalent of the multi-objective optimization problem under uncertainty. Results of credibility, possibility and necessity based FEVM are presented and thoroughly analyzed. PO solutions obtained from possibility based FEVM have the optimistic attitude. Similarly, PO solutions obtained from necessity based FEVM have the pessimistic attitude. This gives a key to decision maker to select any point based on existing risk appetite.
Screening is an important step for dry beneficiation of iron ore. Crushing and screening is typically the first step of iron ore beneficiation processes. In most ores, including iron ore, valuable minerals are usually intergrown with gangue minerals, so the minerals need to be separated in order to be liberated. This screening is an essential step prior to their separation into ore product and waste rock. Secondary crushing and screening can result in further classification and grading of iron ore. The fines fraction is usually of lower grade compared with lump ore.
Hematite and magnetite are the most prominent iron ores. Most of the high-grade hematite iron ores (direct shipping ore (DSO)) are subjected to simple dry processes of beneficiation to meet size requirements. This involves multistage crushing and screening to obtain lump (31.5+6.3mm) and fines (approximately 6.3mm) products. Low-grade hematite ores need to be upgraded to achieve the required iron content, which involves more complicated ore beneficiation processes. The level of comminution required for the low-grade hematite ore is similar to high-grade ores to deliver the same products, lumps and fines. In most cases, the fines product requires additional separation/desliming stages to remove fines containing a high level of clay and other waste minerals.
Although most of the current world iron ore production is represented by hematite ores, the magnetite reserves are significant and the growing demand for steel has opened the way for many new magnetite deposits to be developed. Compared to direct shipping hematite ores mined from the upper regolith, magnetite deposits require significant and different beneficiation, which typically involves grinding of the run-of-mine ore to a particle size where magnetite is liberated from its silicate matrix. The amount of energy required to produce a magnetite product suitable for sale as pellet plant feed is an order of magnitude higher than an equivalent direct shipping lump and fines hematite project.
Due to the depleting reserves of DSO ores and increasing development of low-grade hematite and magnetite deposits, the need for iron ore beneficiation is increasing. Even the DSO ores are requiring a higher level of processing as the depth of existing mines is increasing (below water table) where ores are wet and more sticky, which creates challenges for conventional crushing and screening.
This chapter reviews the current state of iron ore comminution and classification technologies. Firstly, it discusses the most commonly used crushing and screening technologies, including most common flowsheets and a short review of new trends. This is followed by review of comminution circuits and equipment for magnetite ores including most typical flowsheets and advances in comminution technology.
Variations in iron ores can be traced and mapped using cluster analysis and XRD quantification. Paine et al. (2012) evaluated a large number of iron ore samples from an iron ore deposit. Using cluster analysis and mineral quantification, the ores could be classified into defined theoretical grade blocks, which included high grade, high grade with minor gibbsite, high-grade beneficiation, low-grade beneficiation, low-grade other, and waste. As a result, material with a propensity for higher degrees of beneficiation was identified and delimited.
For iron ore beneficiation, the mineral quantities in the ores is essential to establish the degree of upgrading that can be achieved. In a study of the removal of aluminum in goethitic iron ores, mass balance calculations assisted greatly to assess the maximum amount of Al that can be removed without appreciable iron loss, mainly from the goethite. This is shown graphically in Figure 3.6, which shows that 68% of the Al in the sample is distributed in goethite. The goethite also contains 60% of the iron in the sample and cannot be removed. Therefore, if Al is to be removed, only kaolinite and gibbsite can be eliminated without major iron loss, and only as little as 22% of the Al can be removed by flotation or other methods.
Lattice constant refinement can be used to assess the substitution of impurity elements, especially in fine-grained goethite and hematite, as determined by Schulze (1984) and Stanjek and Schwertmann (1992), respectively.
The use of XRD can therefore give a quick assessment of the extent of Al and OH substitution in hematite and the amount of Al substitution in goethite. This was done for five goethite-rich iron ores and is shown in Table 3.4.
Biogenic iron oxides display intimate association with microorganisms inhabiting the ore deposits. In natural sediments, iron oxide particulates are found to occur in close proximity to bacterial cell walls containing extracellular biogenic iron oxides and various biopolymers. Iron-oxidizing and iron-reducing bacteria colonize the biofilms formed on many iron oxide minerals .
Several types of microorganisms growing under extreme environments altering between acidic to neutral pH, aerobic and anaerobic, as well as mesophilic and thermophilic conditions are capable of microbial oxidation of ferrous iron and reduction of ferric iron.
Some examples are Acidithiobacillus sp., Gallionella sp., Leptothrix sp., Leptospirillum sp., and Thermoplasmales (archea). Leptothrix spp. can form FeOOH sheaths around iron oxide minerals through production of exopolysaccharides as a protection mechanism.
Ancient biogenic iron minerals contain biosignatures as in banded iron formations (BIF). Nanocrystals of lepidocrocite on and away from the cell wall of Bacillus subtilis have been observed due to ferrous iron oxidation. Diverse group of Gram-negative prokaryotes such as Vibrio, Cocci, and Spirillum constitute magnetotactic bacteria which synthesize intra- and intercellular magnetic minerals (such as magnetite) and magnetosomes. Several magnetotactic bacteria (living under aerobic and anaerobic conditions) and their magnetosomes have been isolated and characterized from the Tieshan iron ore deposits in China . Microbially induced iron ore formation has been confirmed at Gunma iron ore mine, Japan .
Ubiquitous microorganisms inhabiting iron ore deposits are useful in iron ore beneficiation (e.g., removal of alkalis, silica, clays, phosphorous, and alumina). Because the presence of phosphorous in the iron ore promotes bacterial growth (as an energy source), iron oxide particles having higher phosphorous contents were seen to be colonized by different bacterial cells. Microbial phosphorous mobilization in iron ores has been reported. A polymer-producing bacterium (B. caribensis) has been isolated from a high phosphorous Brazilian iron ore . Microorganisms such as Acidithiobacillus, Clavibacter, and Aspergillus isolated from iron ores are good phosphate solubilizers, because they generate inorganic and organic acids.
Shewanella oneidensis, an iron-reducing bacterium which produces mineral-specific proteins exhibit surface affinity towards goethite under anaerobic conditions. S. oneidenisis are capable of recognizing (sensing) goethite under anaerobic conditions. Shewanella sp. prefers FeOOH and not AlOOH. Such a preferential microbialmineral affinity could be beneficially used to separate alumina, gibbsite, and aluminum silicates (clays) from iron oxides. Microbially secreted proteins are involved in metal reduction. Protein secretion and transport as well as biosynthesis of exopolysaccharides are very important and useful in iron ore transformation. Shewanella putrefaciens, a facultative anaerobic, Gram-negative bacterium can reduce ferric iron oxides and attach preferentially to magnetite and ferrihydrite. Enhanced adhesion of phosphate-utilizing organisms on iron oxides promotes formation of iron phosphate complexes [17, 18].
Magnetite particles formed by dissimilatory, extracellular iron reduction are generally poorly crystallized. Ferrous ions can react with excess ferric oxyhydroxides to form mixed Fe (II) and Fe (III) oxides as magnetite.
BIM of magnetite has been possible in the presence of cultures of Shewanella and Geobacter. Possibility of intracellular deposition of minerals also exists. For example, intracellular iron sulfide formation within cells of SRB such as Desulfovibrio and Desulfotomaculum species has been reported .
Biomineralization brought out by prokaryotes has practical significance in environmental ore deposit formation, mineral exploration through biomarkers, and also in bioremediation of metal-contaminated waters and soils. For example, formation of extensive Precambrian BIF has been attributed to iron-oxidizing bacteria. Biologically formed minerals may be useful as bioindicators on earth and ocean floors.
An example of BCM is the generation of magnetic minerals by Magnetotactic bacteria. Two types of such bacteria are often mentioned, namely, iron oxidetypes which mineralize magnetite (Fe3O4) and the iron sulfidetypes which mineralize greigite (Fe3S4) .
BIF are the largest iron sources distributed globally dating back to about 4 billion years. They contain up to 50% silica and between 20% and 40 % iron and are sedimentary in origin. Main iron minerals such as hematite and magnetite found in BIF are considered to be of secondary origin. Earlier categorization showed domination of carbonates such as siderite and ankerite. It is likely that different mechanisms might have prevailed in BIF .
One traditional model assumed the oxidation of hydrothermal Fe (II) through biotic and abiotic oxidation. Microfossils found in Australia suggested the existence of Cyanobacteria which display various potential biomarker molecules. The presence of oxygen also has been found from the composition of rocks. Formation of ferric iron oxides without oxygen, involving photo-oxidation of ferrous iron by UV radiation has also been suggested. Another recent hypothesis offers direct biological Fe (II) oxidation by anoxygenic phototrophic bacteria.
The presence and nature of minerals of primary and secondary origin in BIF have been widely analyzed. The presence of iron phases such as magnetite, ferrosilicates, siderite, ankerite, and pyrite needs to be considered. Secondary origins of magnetite have been described. Magnetite could have been formed when microbially reduced ferrous iron reacted with initial ferric oxyhydroxides. Oxidation of siderite could also have occurred.
The majority of iron ores that are currently being mined are known variously as banded iron formation (BIF), taconite deposits, or itabirite deposits and were deposited about 2 billion years ago (Takenouchi, 1980). These ores constitute about 60% of the world's reserves. The BIF is a sedimentary rock with layers of iron oxides, either hematite or magnetite, banded alternately with quartz and silicates. The sediments were deposited in ancient marine environments and all were subjected to weathering and metamorphism to a greater or lesser extent.
Prior to enrichment, these sediments normally contained 2030% Fe. Over time, the action of water leached the siliceous content and led to oxidation of the magnetite and enrichment of iron, forming hematite and goethite ore deposits. The grades of the ore and the impurity content varied with the extent of weathering and metamorphism. For example, in tropical and subtropical areas with high precipitation, high-grade deposits that require little or no beneficiation were formed. In temperate climates with less precipitation, the deposits remained as intermediate-grade deposits that require some form of beneficiation. Grade in all deposits tends to decrease with depth due to reduced enrichment by the action of water, and so upgrading is going to become increasingly important as (deeper) mining continues into the future.
The magnetic taconite deposits of the Mesabi Iron Range of Minnesota are typical BIF-type deposits. They contain quartz, silicates, magnetite, hematite, siderite, and other carbonates (Gruner, 1946). They assay about 30% Fe with about 75% of the iron in the form of magnetite and the remainder is largely iron carbonate and iron silicate minerals.
The principal separation in iron ore beneficiation, therefore, is between the iron minerals, hematite and/or magnetite, and silica, principally in the form of quartz. The use of flotation, either alone or in combination with magnetic separation, has been well established as an efficient method for rejecting silica from these iron ores. There are, however, other impurities in some deposits that also require rejection.
Aluminum-containing minerals in iron ore are detrimental to blast furnace and sinter plant operations. The two major aluminum-containing minerals in iron ore are kaolinite (Al2(Si2O5)(OH)4) and gibbsite (Al(OH)3). Some progress has been made in using flotation to separate kaolinite from hematite.
High levels of phosphorus in iron ore attract a penalty because this makes steel brittle. In magnetite, phosphorus is often found in the form of discrete phosphate minerals, such as apatite, which can be removed by flotation. In hematite and goethite ores, however, the phosphorus tends to be incorporated into the lattice of the iron minerals, often goethite. In this case, separation by flotation is not an option. This type of phosphorus contamination needs to be rejected by chemical means.
Besides the BIF deposits, there are also smaller magmatic and contact metasomatic deposits distributed throughout the world that have been mined for magnetite. These deposits often carry impurities of magmatic origin such as sulfur, phosphorus, copper, titanium, and vanadium. While magnetic separation can reject most of these impurities, it cannot eliminate sulfur if it is present in the form of monoclinic pyrrhotite or an oxide such as barite. Flotation may provide an option for reducing the sulfur content of magnetic concentrates when it is present in the form of metal sulfides. It is not an option for oxides such as barite.
Comminution is needed for the liberation of low-grade ores so that the iron content can be upgraded by gangue removal. This necessitates grinding to such a size that the iron minerals and gangue are present as separate grains. But comminution is an expensive process and economics dictates that a compromise must be made between the cost of grinding and the ideal particle size.
Traditionally, grinding has been carried out using rod, ball, autogenous, or semiautogenous mills usually in closed circuit, that is, after grinding, the material is classified according to size with the undersized portion proceeding to the flotation circuit and the oversized portion being returned to the mill. The major benefit of fully autogenous grinding (AG) is the cost saving associated with the elimination of steel grinding media. In the last 20 years, more efficient grinding technologies, including high-pressure grinding rolls (HPGRs) for fine crushing and stirred milling for fine grinding, have provided opportunities to reduce operating costs associated with particle size reduction. A HPGR has been installed at the Empire Mine in the United States for processing crushed pebbles and its introduction has resulted in a 20% increase in primary AG mill throughput (Dowling et al., 2001). Northland Resources operates the Kaunisvaara plant in Sweden, treating magnetite ore with sulfur impurities in the form of sulfide minerals. The required P80 of the ore, in order to achieve adequate liberation, is 40m. This plant uses a vertical stirred mill after AG rather than a ball mill to achieve this fine grind size with an energy cost saving of 35% or better (Arvidson, 2013).
An important part of the comminution circuit is size classification. This can be accomplished with screens or cyclones or a combination of the two. Since cyclones classify on the basis of both particle size and specific gravity, cyclone classification in the grinding circuit directs coarse siliceous particles to the cyclone overflow. In a reverse flotation circuit, these coarser siliceous middlings can be recovered through increased collector addition but at the expense of increased losses of fine iron minerals carried over in the froth. However, if the required grind size is not so fine, then screening can be used instead of cycloning to remove the coarser particles for regrinding and, thus, produce a more closely sized flotation feed (Nummela and Iwasaki, 1986).
Mineral surfaces, when brought into contact with a polar medium (such as water), acquire an electric charge as a consequence of ionization, ion adsorption, and ion dissociation. The surface charge on iron oxides and quartz is accounted for by the adsorption or dissociation of hydrogen and hydroxyl ions. Because these ions are potential determining ions for both iron oxides and quartz, control of pH is important in the flotation of these minerals since the extent of surface ionization is a function of the pH of the solution.
Table 11.1 shows the points of zero charge (pzc's) for some iron oxides and quartz (Aplan and Fuerstenau, 1962). This property is important when using flotation collectors that are physically adsorbed, for example, amines. The pzc's for the three iron oxides, hematite, magnetite, and goethite, are around neutral pH (~pH 7), whereas the pzc for quartz is in the acidic region (~pH 2). The pzc is the pH at which the charge on the mineral surface is zero and is usually determined by some form of acidbase titration. Surfaces of minerals can also be investigated using electrokinetic phenomena with results generally being expressed in terms of the zeta potential. The zeta potential is calculated from measured electrophoretic mobility of particles in an applied field of known strength, and the term isoelectric point (iep) refers to the pH at which the zeta potential is zero. Generally, the iep and pzc are the same if there is no adsorption of ions other than the potential determining ions H+ and OH, but care should be taken with these measurements as evidenced by the variability in the literature regarding the pzc's and iep's of these minerals. For example, Kulkarni and Somasundaran (1976) determined the iep of a hematite sample to be 3.0, but the pzc of the same sample, measured using titration methods, was determined to be 7.1. These results were explained by the presence of fine silica in the hematite sample that influenced the surface properties measured by electrophoresis.
An understanding of the surface properties of minerals is utilized in the selective flotation and flocculation of minerals. For example, consider a mixture of hematite and quartz. The selectivity of the separation between hematite and quartz is related to differences in the surface charge of the two minerals. Below the iep, the mineral surfaces are positively charged and an anionic (negatively charged) collector can adsorb and render the mineral floatable; above the iep, the mineral surfaces are negatively charged and a cationic (positively charged) collector can adsorb and render the mineral floatable. From electrophoretic mobility measurements, the iep's for hematite and quartz are around pH 6.5 and 2, respectively. By choosing the correct collector type and pH, it is therefore possible to selectively float quartz from hematite with dodecylammonium chloride or float hematite from quartz with sodium dodecyl sulfate. This is illustrated in Figure 11.1 (after Iwasaki (1983)). This example is an idealized system, however, and in practice, the presence of slimes and various ions in solution will lead to variations to this model flotation behavior.
Figure 11.1. (a) Electrophoretic mobility of hematite (H) and quartz (Q) as a function of pH; (b) flotation of hematite and quartz with 104M dodecylammonium chloride (DACl); (c) flotation of hematite and quartz with 104M sodium dodecyl sulfate (NaDS) (Iwasaki, 1983).
In this paper, our interests are particles dispersed in a liquid (mainly water), relevant for many industrial particle processing operations. Recently, in-situ synthesis of dispersive nanoparticles has been developed [13,14]. However, there are limitations in the potential combinations of dispersive surfactant molecules and liquids which can be used. In other words, the type of dispersive nanoparticles synthesized by these methods is limited to specific conditions. In this paper, dispersion of fine particles synthesized or generated from natural ores, mainly hydrophilic oxide particles, is discussed. Such oxide particles are processed in plants in diverse fields from pharmaceuticals to natural ore beneficiation by standard separation methods, such as froth flotation, where a surfactant (collector) selectively adsorbs onto a target mineral particle to change its hydrophobicity. Air bubbles injected into the cell attach to the hydrophobic particles due mainly to the hydrophobic interaction, and the particlebubble complexes rise to the airwater interface for collection . This method relies on good dispersion of the different mineral particles from a ground ore in order to have selective attachment of the surfactant onto the target mineral particles. In other words, selective dispersion/liberation is a key to achieving the successful enrichment of the target mineral by flotation [16,17]. Common particle dispersion methods can be divided into two categories: chemical (e.g. pH adjustment (to increase the magnitude of surface charge), dispersant addition); and physical (e.g. agitation, sonication, centrifugation, filtration (to remove fine particles), wet milling [e.g. 18,19]). However, these dispersion methods often have difficulty in achieving selective particle dispersion in concentrated suspensions. For example, wet milling uses a compressive force to break the particleparticle interactions; but it is non-selective (breaking/dispersing all particles regardless of mineral type) and is also energy inefficient [e.g. 20]. Therefore, there is an urgent need for efficient selective dispersion techniques, such as the application of electrical disintegration for fine particle dispersion.
Iron ore is one of the important raw materials for the production of pig iron and steel in the iron and steel industry. There are many types of iron ore. According to the magnetic properties of the ore, it is mainly divided into strong magnetism and weak magnetism. In order to improve the efficiency and production capacity of ore dressing and meet the smelting production requirements of iron and steel plants, appropriate and technology should be selected according to the different properties of different iron ore during beneficiation to achieve better beneficiation effects.
The composition of iron ore of a single magnetite type is simple, and the proportion of iron minerals is very large. Gangue minerals are mostly quartz and silicate minerals. According to production practice research, weak magnetic separation methods are often used to separate them. In a medium-sized magnetic separation plant, the ore is demagnetized and then enters the crushing and screening workshop to be crushed to a qualified particle size, and then fed to the grinding workshop for grinding operations. If the ore size after grinding is greater than 0.2 mm, one stage of grinding and magnetic separation process is adopted; if it is less than 0.2 mm, two stages of grinding and magnetic separation process are adopted. In order to increase the recovery rate of iron ore as much as possible, the qualified tailings may be scavenged and further recovered. In areas lacking water resources, a magnetic separator can be used for grinding and magnetic separation operations.
Because magnetite is easily depleted under the effect of weathering, such ores are generally sorted by dry magnetic separator to remove part of gangue minerals, and then subjected to grinding and magnetic separation to obtain concentrate.
The magnetite in the polymetallic magnetite is sulfide magnetite, and the gangue mineral contains silicate or carbonate, and is accompanied by cobalt pyrite, chalcopyrite and apatite. This kind of ore generally adopts the combined process of weak magnetic separation and flotation to recover iron and sulfur respectively.
Process flow: the ore is fed into the magnetic separator for weak magnetic separation to obtain magnetite concentrate and weak magnetic separation tailings, and the tailings enter the flotation process to obtain iron and sulfur.
The common process flow in actual production is: the raw ore is fed into the shaft furnace for roasting and magnetization, and after magnetization, it is fed into the magnetic separator for magnetic separation.
Gravity separation and magnetic separation are mainly used to separate coarse-grained and medium-grained weakly magnetic iron ore (20~2 mm). During gravity separation, heavy medium or jigging methods are commonly used for the gravity separation of coarse and very coarse (>20 mm) ores; spiral chutes, shakers and centrifugal concentrators for medium to fine (2~0.2mm) ores, etc. Reselect method.
In magnetic separation, the strong magnetic separator of coarse and medium-grained ore is usually dry-type strong magnetic separator; the fine-grained ore is usually wet-type strong magnetic separator. Because the grade of concentrate obtained by using one beneficiation method alone is not high, a combined process is often used:
Combination of flotation and magnetic separation: the magnetite-hematite ore of qualified particle size is fed into the magnetic separator for weak magnetic separation to obtain strong magnetic iron ore and weak magnetic tailings, and the tailings are fed into the magnetic separator for weak magnetic separation. In strong magnetic separation, strong magnetic separation tailings and concentrate are obtained, and the concentrate is fed to the flotation machine for flotation to obtain flotation iron concentrate tailings.
Combined gravity separation and magnetic separation: similar to the combined flow of flotation and magnetic separation, only the flotation is replaced by gravity separation, and the products are gravity separation concentrate and tailings. These two combined methods can improve the concentrate grade.
The above are mainly the common separation methods and technological processes of strong and weak magnetic iron ore. The composition of natural iron ore is often not so simple, so in actual production, it is necessary to clarify the mineral composition, and use a single sorting method or a joint sorting method according to the corresponding mineral properties. Only in this way can the beneficiation effect be improved.
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The ore is again screened, with finer particles falls into the screens ceramic lined underpans and coarse particles are conveyed via anover belt magnetic separatorto a Dense Medium Separation Drum. It is then laundered with aceramic linedlaunder before again being screened by coarse waste and coarse iron drain and rinse screens. Refined coarse product can be sampled with ahammer sampler.
The ore from the desliming screen is sent to aDMS cyclonewhere separation sends some of the ore throughsievebend housingsand the more valuable stream to static screens. Waste and coarse material from both these screens go to the magnetic separator, while the balance, medium product, is diverted for use.
A portion of the ore from the desliming screen is sent directly to a sink drain & rinse screen and a jig screen, which separates particles into different quantities based on density. After screening, the waste stream is sent to a floatation screen resulting in waste product, and the mineral rich stream is sent to a sink screen, subsequently producing additional medium product.
As a technology-driven company, our iron ore beneficiation solutions are supported by a team of skilled engineers and metallurgists, with a wealth of process and application knowledge that we use to achieve gold beneficiation at the lowest cost per ton.